The present invention relates to an optical feeler for monitoring the reserve of thread in devices for feeding weft to looms and textile machines in general, and to the weft feeder that includes such device.
More particularly, the present invention relates to conventional weft feeders comprising a fixed drum on which a windmilling arm, actuated by an electric motor, winds a plurality of spaced turns of thread which constitute a weft reserve, wherein the turns of the reserve are unwound from the drum in a preset amount at each beat of the loom and wherein optical feeler means are provided and are suitable to start and stop the motor that actuates the windmilling arm when the weft reserve drops below a preset number of turns and respectively when the reserve is fully replenished or if the thread breaks.
Conventional optical devices for monitoring and detecting the presence of the turns of reserve thread are based on the use of one or more light-emitting devices which direct the light rays onto the thread wound on the drum and of one or more receivers which collect the rays and convert them into electric signals used to indicate the presence or absence of the spaced turns of thread. In the disclosure, the expression "light rays" refers both to rays within the visible-light spectrum and rays within the invisible-light spectrum, for example infrared rays.
In particular, prior European patent No. 624.674 discloses a conventional device for monitoring the weft reserve and the breakage of the weft thread in weft feeders, which comprises an emitter diode and a receiving phototransistor which are arranged at the same upper face of a transparent plate which lies on the drum of the weft feeder and receives the turns of the weft reserve. The emitter sends onto the transparent plate a beam of rays which is inclined by a preset angle of incidence and the receiver receives a beam of rays which emerge from the transparent plate and are inclined by an angle of reflection which is different from the angle of incidence and is produced by multiple-reflection means arranged on the other lower face of the transparent plate. The incident and emerging rays are affected by a turn of the thread reserve when the turn reaches the plate, so as to block out the receiver, which emits a corresponding electrical signal indicating the presence of turns.
Prior European patent No. 327.973 discloses a device for monitoring the weft reserve which is based on the use of a pair of photoelectric cells which send respective rays of light, concentrated by lenses, onto a reflective surface of the drum on which the turns of the reserve are wound. The cells generate electrical signals which are produced by the reflection of said rays. The electrical signals have different durations depending on whether the turns of thread that intersect the incident and/or reflected light rays unwind from the drum by being requested by the loom or advance along the drum by being wound by the windmilling arm in order to form the weft reserve. An electronic circuit, connected to the photoelectric cells, is provided in order to discriminate the signals produced by the advancement of the turns that form the reserve from the signals produced by the unwinding of the turns that leave the drum by being requested by the loom, so that the signals are correctly used to control the actuation motor.
It is also known to perform the optical detection of the presence of the turns of thread of the weft reserve by detecting, by means of a photosensor suitable to provide a useful electrical signal, the light rays reflected by the thread and emitted by a light source which is constituted by a photodiode or the like.
Another known method, disclosed for example in prior Application WO 91/18818, uses an array of sensors, for example optical sensors, arranged on the drum at respective turns of the reserve and suitable to provide signals which are processed by electronic circuits in order to provide various information, including information regarding the presence or absence of the turns of thread.
The above-mentioned conventional monitoring systems entail some drawbacks which limit their use and diffusion. Generally speaking, the systems in fact are all relatively complicated and expensive, and those based on optical reflection entail the drawback that their functional characteristics change over time, due both to the accumulation of dust, lint and the like and to the possible variation of the reflection coefficients of the reflective surfaces, caused by wear and/or microstructural alteration of the materials.
All the above-mentioned conventional systems further have, to varying extents, a sensitivity to ambient light which can bias the monitoring signals and thus requires the use of sophisticated electronic circuits for processing such signals.